PRRS was recognized for the first time in the United States in 1987. It quickly spread throughout all of the major swine producing areas of North America. It next appeared in Europe, and today PRRSV has almost worldwide distribution. Many swine producers, government officials, and veterinarians believe that PRRS is currently one of the most serious economic threats faced by the swine industries worldwide.
As its name implies, PRRS is characterized clinically by its ability to cause reproductive failure in pregnant females, especially when initially infected late in gestation, and respiratory tract illness in pigs of all ages, but most common and severe in young pigs. A PRRSV infection is also thought to potentiate the effects of other swine pathogens. On the basis of retrospective serological studies, it also has become evident that many infections of swine with PRRSV are either subclinical or result in less obvious clinical signs. Therefore, the PRRSV often gains access to a herd and spreads extensively before its presence is first detected.
The virus can persist in an infected host for at least several months. Such “carriers” perpetuate the infection and make control of the disease extremely difficult. As a consequence, the most effective means for reducing the economic impact of PRRSV is to vaccinate (immunize) potentially susceptible pigs before they are exposed to virulent field virus.
Attenuated vaccines, (manufactured by Boehingher Ingelheim) prepared from single strains of PRRSV, are commercially available. One is licensed for use in pigs between 3 and 18 weeks of age for the prevention of respiratory tract illness (Gorcyca et al., 1995). One is licensed for pre-breeding.
Another attenuated vaccine has been described for the prevention of reproductive failure (Hesse et al., 1996). It is prepared from a single strain of PRRSV and has only been tested against a single strain of PRRSV. The challenge strain is described as heterologous on the basis of the anamnestic response of vaccinated gilts following challenge; however, no other evidence has been presented to establish that the two strains, i.e., the one used for the vaccine and the one used for challenge of immunity, are genetically or antigenically different.
There are two known major serotypes of PRRSV (Done et al., 1996). One (prototype Lelystad) is representative of at least most strains that have been isolated in Western Europe. The other (prototype ATCC 2332) is representative of at least most strains isolated in North America and Asia. There also are antigenic variants within prototypes (Meng et al., 1995), and base sequence differences among strains isolated in North America have allowed for their differentiation (Wesley et al., 1996).